Transistor amplifier hearing aid unit with receiver vibration feedback suppression



May 24, 1960 F A. HERRMANN 2,938,083

TRANSISTOR AMPLIFIER HEARING AID UNIT WITH RECEIVER VIBRATION FEEDBACK SUPPRESSION 4 Trae/Vix!" May 24, 1960 F. A. Hr-:RRMANN 2,938,083

TRANSISTOR AMPLIFIER HEARING AID UNIT WITH RECEIVER VIBRATION FEEDBACK SUPPRESSION Filed Dec. 30. 1957 2 Sheets-Sheet 2 EES.

tarios Frank A. Herrmann, WhiteA Plains, Sonotone Corporation, Elmsford, of New York Filed Dec. 30, 1957, SerflNo. 705,841 Claims. (Cl. 179-107) N.Y., assignor to N.Y., a corporation This invention relates to all-in-one unitary transistor hearing aids wherein all its elements including the micro-4 phone, the transistor amplier, the receiver or earphone, and the battery cell, are combined into a single miniature# size unit of an over-all volume, small enough to tit within the outer ear cavity of the users ear. Since suchv hearing aids have to operate with a gain of about 30 decibels, plus-minus `5 decibels, it is essential to prevent feedback` to the microphone of thesound output of theearphone, as well' as of the incident corresponding mechanical' reaction vibration of the earphone bodyor casing, while secur, ing the desired minimum over-all volume of such hearing, aid unit. Y

AIn accordance with the invention, the required essential reduction ofthe vibration feedback from. the earphone body to the microphone is obtained'by joining the amplilier-microphone section thereof to its tiny earphoneby a. body of vibration-attenuating elastomer material. of suficient volume and sound dissipating, characteristicsto, secure the required suppression of feedback of mechani cal vibrations from the earphone body to the amplitier-.-

microphone section.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof, reference being had to the accom-- panying drawings wherein:

Fig. 1 is an elevational view, with parts in section, of one form of a unitary hearing aid unit exemplifying the. invention, as it is held in its operative position within the` outer ear cavity of the user;

Fig. 2, is a top View of the hearing aid unit of Fig. 1;

Fig. 3 is a circuit diagram of the 1 and 2;

Figs. 4 and 5 are typical frequency response curves ofal microphone and earphone, respectively; and l Fig. 6 is an elevational view showing how a hearing aid unit similar to that or Figs. 1 to 3 is worn behind theV ear at its junction to the adjacent head body portions.

Referring to Figs. 1 and 2, the hearingv aid unit gener-ally designated 10 hassuch small lateral dimensions that its outer Wider amplifier section 30 tits withinv the concha4 cavity 11 of the users outer ear between the tragus :12 and anti-helix 13 thereof. The receiver or earphone 21 has a sound outlet duct 22 ear tip 23 of elastomer material, such as rubber, iittng Within and acoustically sealing the ear canallS of the user.'- 'For the sa-ke of clarity, the partsA of the -uses earl including its tragus 12, anti-'helix' 13 and the walls of its ear canal 15, are indicated in dash-double-dot lines.

. The hearing aid of Figs. 1 and 2. embodies and combines the elements and components shown inthe circuit diagram, Fig. 3. it comprises a microphone 31 the output of which is impressed on anamplier operating with three transistors 32, 33, y34, which deliver the amplified microphone output to the receiver or earphone "2i, Thecrcuits ofall three transistor amplifierl stages are ,energized from the same energy source consisting of a single, tiny battery cell 35 which may be connected or disconnected hearing aid of Figs.`

connected through a flexible festem outer space is transmitted to the microphone for causing it' the present application.

from the transistor amplifier' stage by a switch 36. The microphone 31 and the receiver 21 have electromagnetic sound energy transducers. For instance, the microphone 31 may have a transducer of the type'shown in Egerton Patent 1,385,898, and the earphone may be of the type shown in Pearson Patent 2,493,734. The output of the microphone. is delivered to the base and emitter of the iirst transistor 32 through a circuit including a by-pass capacitor 41 and resistances 42, 43. Direct-current bias for transistor 32 and for the other transistors is provided by voltage-dividing,resistances 44, y45 which are connected through energy supplyleads yd6, 47 to the opposite terminais of the supply cell 35. The two resistances 44, 45

have an intermediate tap 48 from which the proper bias current is'y impressed between the base and emitter of transistor 3'2, through a bias circuit including resistance 49 connected between the emitter of transistor 32 and supply lead 47.

The output developed between the collector and emitter of the rst transistor 32 is impressed between the base and emitter of the second, transistor 33. The emitter of transistor33 is connected to its supply lead 47 by a resistance element 5'1 which is by-passed by a capacitor 52,. The collector of the second transistor 33 is connected through load resistance I55 to supply lead 46. The base and emitter of these transistors have applied thereto proper bias by thevoltage-dividing resistance elements y44, 45 and intermediate tap l48. The output developed between the collector and emitter of transistor33 is delivered through a circuit including by-pass capacitor '56 to the base `and emitter of the third transistor Proper bri-as is applied by the voltage-dividing resistance elements.44,`-45 to the base and emitter of transistor 34 through a circuit includ-4 ing resistance I57. The output developed between the collector. and emitter of the third transistor 34 is` delivered.

to the winding of the receiver 2l for-energizing it with amplified current sufficient to cause its diaphragm to deliver to the ear canal of the user the required amplifier. sound output. corresponding to the desired amplification of 25 to 35 db.

Referring to Figs. 1 yand 2, all of the foregoing circuit components and elements described above in connection with Fig. 3, except the earphone, are combined into the wider amplifier section 30 having an over-all volumeto permit insertion thereof into the outer cavity of the users ear, with lateral dimensions fitting between the anti-helix tragus and anti-tragus. The ampliiier unit 3u has a chassis plate 61 along which are assembled all the circuit ele-- ments of the `amplifier shown diagrammaticallyin Fig. 3

With the electromagnetic transducer element of the rc-` ceiver 21,. Most of the other ampliiier components andl elements are carried along the underside of the chassis.

The amplier Chassis plateplatel, as shown in Fig. 1. 6:1 with theamplier elements and microphone mounted along it, are housed in a metallic casing e3 having an' outer casing Wall '64 overlying the microphone 31 and provided with openings 65 through which sound from the to' generate ,corresponding electric output impressed on the transistor amplifier. In general, the amplifier arrangement is of the type described in co-pendinglapplication's. Serial Nos, 615,189, tiled'October, 10, 1956, and 634,368, assigned to the assignee o@ tiled January, 15, 1957, both except for the receiver 21. The chassisl plate 61 consists; of a printed circuit member ot' insulating material pro The microphonel as shown in Fig. f magnetic shield The upper amplier-casing wall 65 has an opening 66 through which the battery cell 35 may be inserted between a fixed terminal member 67 carried by the chassis plate 61 and a llexible terminal member 68 having a grip iinger 69 which may be gripped on the exterior of the casing cover 65 for releasing the battery from its operative position in the -ampliiier section 30. Upon gripping the projecting iinger 69 of the battery holding spring 68 and ilexing it to the right, as" seen in Figs. 1 and 2, the battery cell 35 is free to drop out from the battery compartment of the amplifier casing 63 forreplacing it by another cell. t

The receiver 21 has a light vibratory diaphragm 25 which is vibrated by a receiver body 26y for generating the required sound output which is transmittedV through the receiver sound outlet duct 22 and its exible rubber ear tip 23 to the ear canal of the user. r These vibrations of diaphragm 25 cause it to exert opposite inertia reaction forces on the receiver body 26, which as' a result, undergoes similar vibrations of much smaller amplitude, corresponding to its larger mass. These inertia reaction vibrations of the receiver body 26 are transmitted to any other body which is mechanically joined thereto. Thus, when the receiver or earphone 21 is directly attached to the body of the amplifier-microphone section, the minute mechanical vibrations of the receiver body 26 will be transmitted to the casing 63 of the microphoneampliiier section 30 and these casing vibrations will in turn be transmitted mechanically to the microphone 31. In addition to the mechanical receiver-body vibrations so transmitted to the microphone 31 by the amplifier-microphone casing 63, the large area of the so-vibrating casing 63 excites the surrounding air into corresponding vibra. tions which will be acoustically transmitted to the micro-V phone. In the past, diiculties have been encountered in finding a way for suppressing mechanical and acoustic feedback of the minute mechanical vibrations from the earphone body 26 to the adjoining ampliiier-microphone casing of such unitary hearing aid when it is` operated at the desired full gain of 25 to 35 db.

The present inention overcomes the diiiiculties connected with suppression of acoustic and mechanical vibration feedback from the earphone body to the adjoining microphone-amplilier section of such hearing aid unit. In accordance with the invention, excessive feedback action of the minute mechanical vibrations of the earphone body 26 are suppressed by connecting the earphone 21 to the amplifier-microphone section 30 through a relatively large barrier body 80 of sound-absorbing and sound-attenuating elastomer material, such as rubber. In the form shown, the elastomer barrier body 80 has a body portion atlixed to the adjoining wall portion-67 of the amplifier-micro-` phone casing 63, which is separated by a relatively thick intermediate barrier body portion 82 from a relatively remote barrier body part 83, which is secured to the vibrating earphone body 26, so that the elastomer material of the substantially thick intermediate barrier portion 82 is interposed between the earphone body 26 and the amplifier casing portion 66, for suppression of mechanical vibration feedback to casing 63 and the microphone 31 heldtherein.

In the form shown, the barrier elastomer body portion 81 is suitably joined to the metallic casing wall portion 66 of the amplifier-microphone unit 30 as by cement.V The remote barrier elastomer portion 83 forms an elastomer body projecting a distance away from the amplilier casing 63 and separating therewith the earphone body 26, with the intermediate elastomer body portion 82 having suiiicient volume and vibration-dissipating characteristics for keeping feedback of mechanical earphone body vibrations below a level that would set up generation of sustained electrical and mechanical oscillations and continuous earphone whistling.

The relatively large mass of intermediate rubber bar. riet' body portion 83 forms a collar holding the earphone body 26 spaced from the adjoining amplifier casing portions 36, 56. The rubber barrier body portion 80, which is secured tothe ampliiier casing portion 67, has openings through which insulating conducting leads 27 of the earphone are connected to the outward side of the amplifier housed in the amplier casing.l With the arrangement of the type shown and, described, the earphone body portion 26 to'which the remote rubber body portion 83 is secured, is of smaller lateral dimension and is surrounded by the rubber barrier body 80, thus exposing only a small area of the vibrating earphone body to the surrounding air and thereby Vreducing acoustic air feedback of the minute mechanical vibrations of the earphone body to the microphone. y

Fig. 4 shows a curve diagram typical of the frequency response of the microphone 31 of the hearing aid unit of the type described above in connection with Figs. 1-3, and Fig. shows a curve diagram typical of the response of an earphone of'such hearing aid unit. Fig. 4 shows that the vresponse of the microphone rises from about 300 c.p.s. (cycles per second) at a rate of about 6 decibels per octave to 1500 c.p.s., the response thereafter continuing at about the same raised level with an anti-resonant peak at about 2200 c.p.s. and a resonant peak at about 3500 c.p.s., with the response dropping sharply at about 4000 c.p.s. Fig. 5 shows that the response of the earphone is substantially level from 100 c.p.s.k to 1000 c.p.s., rising thereafter at a rate of S decibels per octave to 2000 c.p.s.,

and dropping thereafter back to about the same level, as

around 100 to 1000 c.p.s., and dropping sharply at 3500 c.p.s. The earphones o f the type used in connection with the hearing aid of Fig. 6 have a frequency response similar to that shown in Fig. 5, except that it 'remains at about the same level between 100 and 3500 c.p.s., with l2 db peak at 2000 c.p.s.

The vibration-dissipating barrier body 80 dissipates only' very little of the low-frequency vibrations below 1000 c.p.s. which are transmitted from the body of the earphone 21 to the ampliiier casing 30. However, since the microphone has a relatively low sensitivity in this low-frequency range below 1000 c.p.s., the microphone will not be sufliciently excited by the transmission of these low-frequency earphone body vibrations to cause the hearing aid unit to become self-excited and set up whistl-` ing oscillations. On the other hand, earphone body vibrations in the frequency range from about 1500 c.p.s. and higher, if transmitted through the ampliiier casing 63 to the microphone 31, would be sutcient to set up in the hearing aid unit of Figs. 1 3, continuous oscillations and whistling. The vibration-dissipating barrier body 82 of the type described above in connection with Fig. l, has sulicient vibration-disspating capacity for vibrations from 1000 c.p.s. and higher to suppress transmission of such 'vibrations from the body of the earphone 21`to the amplier casing 30, and keep their trans-2 mission below a level which would set up self-sustaining oscillations and whistling in the hearing aid unit of Fig.

l. Although it has such desired earphone vibration-dissipating characteristics, the barrier body may vbe designed to provide a relatively positive connection between the microphone ampliiier section 30 and the earphone 21 so that they form a self-supporting unit.

Without thereby limiting the scope of the invention, there aregiven below data of a commercial form of hearing aid exempiityingthe invention:

The transistors are of the PNP type.

The casing of the ampliiier unit has lateral dimensions by 9', and a totalheight of l/", the two complementary .parts off the casing being of stainless steel.

The battery cell 35 has a maximum outer diameter of 7A6, and a thickness of Ms".

The earphone 2-1 has a circular shape. Its outer pordiameter of and an axial height of 5/2". The inner,

narrower, receiver earphone body portion 26 is @i6 in diameter, and %2 high.

The part of the receiver casing has a stainless steel exterior.

The elastomer barrier body 30 is of silicon rubber and the weight of its mass is .02 ounce. lts compliance is on the order of .0001 inch displacement by a force of one ounce. The total height is about 1A. lts Wall portion Si has a square shape about 1/2 by 1A." in size, with a wall thickness at its outer region of .060. The collarshaped part of the intermediate elastomer body portion 32 is of circular shape; it has an outer diameter of 1/2, and its cross-sectional surface corresponds to an area of 1/8" X ll.

ln Fig. 6 is shown a hearing aid unit similar to that of Figs. l5, wherein the sound duct Z2 of the receiver 2i has connected thereto a small, hollow tubing 85 of flexible material such as neoprene, which extends to an eartip 23 of iiexible rubber-like material, such as described above, seated and held in the ear canal of the user for transmitting the earphone sound output to the users hearing organs. The flexible, sound-conducting tubing S5 which extends from the ear tip in the ear canal of the user is looped in over-lapping engagement over the upper junction portion of the auricle to the head, and extends along the rear of the ear to the sound outlet duct 2v2 of the earphone 26, so as to carry and support it together with the other parts of the unitary hearing aid unit similar to that of Figs. l and 2, behind the users ear. The hearing aid unit 9d of Fig. 6 is otherwise similar to that described above in connection with Figs. 1 to 5.

As used in the specification and claims herein, the expression suppress is intended to mean attenuation of an undesirable action to a degree sucient for securing the desired over-all effect.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specilic exempliications thereof, will suggest various other modications and applications of the same. lt is accordingly desired that in construing the breadth or the appended claims they shall not be limited to the specific exempliflcations of the invention described above.

l claim:

1. in a hearing aid, an earphone; an amplifier section including a microphone, a battery cell, and a transistorampliiier including at least one transistor electrically connected to said cell and between said microphone and earphone for supplying amplied microphone output to said earphone, said ampliier section constituting a unit separated from said earphone; said microphone having a materially lower response at frequencies below 1000 cycles per second than at a frequency between 1500 and 4000 cycles per second; and a barrier body of rubber-like yieldable elastomer material Iforming the sole mechanical adjoining the eartip 23 connection joining said earphone and said amplifier section into a selfsupporting structure movable as a unit relatively to the ear of the user, which structure has an over-all volume corresponding to an object itting into the outer cavity of the ear between the anti-helix, tragus and anti-tragus; said barrier body having the property of materially attenuating vibratory energy of Ifrequencies higher than 1000 cyclesper second, a body volume of said barrier body which is of substantial thickness separating said earphone and said amplifier section for suppressing feedback of mechanical vibrations of high frequencies of at least `about 1000 cycles per second from said earphone to said amplier section and thereby preventing the setting up of sustained electric oscillations and earphone Whistling at such high frequencies.

2. In a hearing aid as claimed in claim 1, said amplifier section having a casing enclosing at least a major part of the `spaced occupied thereby, said barrier body forming the sole junction earphone.

3. In a hearing aid as claimed in claim l, said ampliier section having a metallic casing enclosing at least a major part of the space occupied thereby, said barrier body forming the sole junction between said casing and said earphone.

4. In a hearing aid as claimed in claim 1, said earphone being positioned eccentrically relatively to the nearby portion of said amplifier section and forming therewith a self-supporting structure shaped and of a volume small enough to lit and be held within the outer ear cavity of the user between the anti-helix, tragus and anti-tragus, said amplifier section having a casing enclosing at least a major part of the space occupied thereby, said barrier body forming the sole junction between said casing and said earphone.

5. In a hearing aid as claimed in claim 1, said earphone being positioned eccentrically relatively to the nearby portion of said amplier section and forming therewith a self-supporting structure shaped and of a volume small enough to lit and be held within the outer ear cavity of the user between the anti-helix, tragus and antitragus, said amplifier section having a metallic casing enclosing at least a major part of the space occupied thereby, said barrier body forming the sole junction between said casing and said earphone.

between said casing and said References Cited in the le of this patent UNITED STATES PATENTS 2,787,670 Rowland Apr. 2, 1957 2,824,177 Tado Feb. 18, 1958 FOREIGN PATENTS 922,889 Germany Ian. 27, 1955 

